In a fuel cell system that generates electricity using an electrochemical reaction between fuel gas (hydrogen) and oxidation gas (oxygen), conditions to control an auxiliary device that supplies hydrogen and oxygen are set so that a predetermined output can be obtained from a fuel cell, having a problem that conductivity of electrolyte and activity of electrode catalyst, for example, are degraded over time, thus leading to a failure to obtain the predetermined output from the fuel cell.
To cope with this, a device is proposed, which diagnoses a degradation state of a fuel cell based on a current/voltage characteristic and corrects conditions to control auxiliary devices based on a result of the diagnosis so that a system efficiency after degradation can be maximized (see JP-A-2004-164909).